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Medical Metrology Finds the Best Fit

There are a number of ways of measuring and inspecting biomedical devices. Which way is best?

By Bruce MoreyContributing Editor

Finding the best fit of device to part is especially significant for metrology companies with a diverse portfolio. That is the case for Mitutoyo (Aurora, IL), according to Allen Cius, their vision optics manager. The company offers over 6000 products, from hand calipers to horizontal arm CMMs that can measure objects the size of whole car bodies. Mitutoyo metrology capital equipment systems fit within three basic families of products: vision-based systems, form measurement machines, and CMMs. “Vision systems can very accurately measure 3D with camera and high-resolution Z scales,” said Cius. Form devices use dedicated devices to measure surface roughness, roundness, or profiles. CMMs are CNC-driven three-axis devices that use probes or specialized sensors to measure parts according to programs. However, Cius noted, these distinctions are blurring. “These platforms are expanding in capability,” he said. CMMs are now outfitted with contact probes, line laser scanners, and even surface roughness probes. Vision systems use laser scanners that collect thousands of points per second, in addition to touch and scanning probes.

“It is often the case that we start with the piece that needs to be measured and after some experimentation, decide what is best,” he said. Cius offers the following list of factors that should be considered:

Does the part require contact or noncontact sensing? Size, flexibility, and fragility of the surface influence this parameter.

What accuracy is required? This may be process dependent as well, for instance, less accuracy may be required to maintain a production line, while more is needed during design investigation or first-article qualification.

What density of measurements is needed? Are a few critical characteristics enough, or are surface or contour measurements required?

What is the anticipated volume of measurements? Will R&D researchers be using the device, or is to be used in controlling manufacturing processes on the shop floor? A hand-held device may be good for one, while an automated, “lights-out” system is the only solution for the other.

Cius said that biomedical parts that are now measured with Mitutoyo vision measuring systems include orthodontics, angioplasty inserts, stents, hearing aids, and GI bag-clamps. Most of the time with medical applications, research leads to the selection of a vision measuring system, often Mitutoyo’s Quick Vision vision-based systems. The wide range of options and variety of sensors now available for the Mitutoyo Quick Vision means a solution can be found for most any part, from simple to complicated. Sensors outfitted on the Quick Vision foundation besides the 2D camera include touch probes, structured light interpretation, lasers, and a new chromatic sensor.

Vision Systems Popular with Device Makers

Another supplier of multiple metrology systems is Carl Zeiss Industrial Metrology (Brighton, MI). “Zeiss is one of the few one-stop shops for your medical inspection needs,” said Kevin Legacy, business manager for Metrology Services for Zeiss. He points to three measuring machines suitable for medical implants: the Metrotom, O-Inspect, and Micura. Companies using more than one device can benefit from the fact that Zeiss’ Calypso software drives each. “A module of Calypso called Master Control Center provides tools to manage two important FDA criteria,” he said. The first is revision control of CMM inspection programs. The second is compliance to CFR Part 11, which covers the trustworthiness of electronic records and signatures.

For measuring inside parts nondestructively, or for parts that are flexible or transparent, Legacy recommends the Metrotom Computed Tomography CMM. The system exploits X-rays for metrology, with recent improvements in measuring volume and combining data scans that he believes benefits medical molding. For ultra-precise, small medical parts where tolerances are ≤ 5 µm (0.0002"), he recommends the Micura CMM outfitted with a VAST XTR scanning analogue probe that provides a 4th axis. It boasts a maximum permissible error of MPEE = 0.8 +L/400 (µm). Finally, for shop-floor use, he recommends the Zeiss O-Inspect. “It offers the ability to combine inspection tasks,” he explained. “A white-light sensor is used for measuring clear materials. Blue and red lighting combined with backlighting is used for high contrast needs and tactile scanning for optimum accuracy.”Jamie Murray, senior applications engineer at Optical Gaging Products (OGP; Rochester, NY) also notes that a vision-based system tends to be the most popular with their biomedical device customers. OGP’s SmartScope family includes three lines of multisensor vision systems—Flash, ZIP, and Quest. Each product line features an optical system and optional sensors to serve a range of part and feature sizes, with appropriate accuracy and precision. SmartScope Quest is a popular solution for medical devices, according to Murray. “The TeleStar optical system is fully telecentric throughout its range which is critical for high accuracy on many types of medical parts.” What attracts biomedical device manufacturers is the range of sensors available on the Quest, including conventional touch trigger probe, scanning analogue probe and the patented TeleStar-Plus interferometric TTL laser. Quest also offers a range of micro-probe options for measuring very, very small features. Measuring envelopes for the Quest family start at 300 × 300 × 250 mm with accuracies of (1.5 +5L/1000) µm.

Murray also reports that the combination of fully telecentric optics and a high-performance interferometric laser are ideal for measuring biomedical devices. “Medical implants tend to be rounded and shiny—characteristics that make them difficult to image accurately with conventional optics. Stem tibia, knee, and hip implants, stents, or highly polished bone plates with rounded edges,” he said. Profile and other Geometric Tolerances define the complex and compound curves of orthopedic implants. These require a dense data set, which in turn requires analysis software to match it to the original CAD data, Murray said. OGP offers SmartProfile software to aid in the analysis. Other types of medical parts, such as molded plastic items, microelectronics, and even eyewear present different sorts of measurement challenges. Syringes and surgical stapler components, for example, require special illumination sources and fixturing to reliably image or provide access to critical features.

Murray also reports that medical customers choosing multisensor vision-based systems usually ask for three sensors. These include their telecentric vision system, a TeleStar TTL laser, and touch probe.

General Systems, Shop-Floor Applications

For companies with fewer, more advanced metrology devices in their portfolio, finding the right niche within the industry sometimes requires educating their potential users. “The medical device community is getting more interested in 3D scanning,” said Pierre Aubrey, President of ShapeGrabber (Ottawa, ON, Canada). The company specializes in 3D laser line scanners. “We are still in the early days in terms of adoption in the medical industry. These early adopters are using ShapeGrabber scanners because their requirements are so pressing they have to turn to a fast laser system.” The strength of systems like their Ai310 is speed and data density—the ability to collect a million points within a few seconds to about a minute. “These are good for complex, curved shapes such as you find in orthopedic implants, ergonomic tools, and medical enclosures and housings,” said Aubrey.

Aubrey noted that their 3D laser scanners have resolutions down to 2 µm and accuracies down to 16 µm (ISO 10360 method), making them ideal for items with unusual shapes with reasonably tight but not extreme accuracy requirements. A number of biomedical devices, from pace makers to implantable drug-delivery pumps have intricate, small parts which may have very tight accuracy requirements. These small parts are typically enclosed in a housing or attached to a frame. “The ShapeGrabber Ai310 automated scanner is ideal to measure such housings. Parts inside the enclosure—flat gears, valves, and other prismatic parts—are probably best measured using other sensor modalities that can attain tighter tolerances,” he said.

Another category of often-used metrology equipment is the venerable Optical Comparator, recently upgraded into an all-digital version by VISIONx (Pointe-Claire, Quebec, Canada distributed in North America by Methods Machine Tools Inc.; Sudbury, MA). The company’s VisionGauge system uses the part’s CAD data to produce a high-contrast image for comparison, eliminating the need for Mylar overlays. The company also claims that it is more accurate, enables faster measurement, and has a smaller footprint compared to traditional optical comparators. “Our system shines where tight tolerances need to be measured on complex geometries and where there are many small-lot productions because there is virtually no setup time,” explained Patrick Beauchemin, president and CEO for the company. “That describes many medical manufacturing applications.” Measured accuracies are as fine as 0.0001" to 6 σ, Beauchemin said. An optional laser is offered for depth and height measurements.

“Where we are seeing the most enthusiastic adoption of our system in the medical industry are bone screws and larger implants, such as knee and hip replacements,” Beauchemin said. “Knees and hips have both plastic and metal, and on traditional optical comparators the plastic surfaces have glare that our system completely eliminates.” A large depth of field on their system, up to 4" (100 mm), allows them to measure bone screws along the helix angle, the preferred method. “You might measure it 12° towards and away from the camera and we can focus on the whole bone screw at once,” he said. Craniomaxillofacial components—complex curved implants for face and head applications—are another important component of his business.

Medical devices implanted in the body tend to be small. CNC machines make many of them and a growing trend is to measure parts directly on a CNC machine to provide control for in-process machining. This would then require on-machine probes that are small as well. The new M&H subcompact infrared 40.50 probe from Hexagon Metrology (North Kingstown, RI) seems ideal. “Medical manufacturing can be very complex, and the more complex the geometry of the part, the more valid this probe is,” said Adrian Johnson product manager for Hexagon Metrology. The entire probe is the size of a thumb drive. Suitable for three-, four-, or five-axis machining, it boasts a repeatability of ±1 µm to 1 σ. “This probe is good for any part that has a free form, compound surface with an organic shape. Orthopedics, for example, such as knees, hips.” He also notes that this type of process is best in repetitive, high volume applications.

Targeted Applications

There is a new trend to build specialized, purpose-built machines that excel at one application. One example is the Marposs Orthoflex system. “We recognized that there was a problem in the super-finishing operations of certain orthopedic implants, such as hip, shoulder, and knee joints,” said Luca Trevisani, technical manager for Marposs (Auburn Hills, MI.) These biomedical implants cannot tolerate any surface scratch or defect since a perfect finish is vital to longevity. “These implants are expected to last many years in someone’s body but those tiny defects affect their lifetime,” he said. Today, humans inspect them manually at end-of-line, rejecting those with defects. “Sometimes, bad parts are not caught during the production process, and they will be inspected and rejected at the hospital prior to an implant,” he said. Rejection by a doctor is, according to Trevisani, a significant issue for both a hospital and supplier.

To move away from a subjective process with potential bias influenced by an operator, Marposs developed the Orthoflex automatic inspection system. The heart of the system is a visual system that collects data as it scans the surface of super-finished spherical shoulder and hip joints. Scratches, nicks, and other surface defects are identified and visualized on a screen and a report is produced that is uniquely identified with each implant’s serial number. The system currently works for the spherical portions of shoulder and hip implants, while knee inspection would require further development, according to Trevisani. Introduced four years ago worldwide, the company reports that acceptance by companies used to end-of-line inspectors is slow but steady. “The difference with Orthoflex is that once it is set with proper reject thresholds it does not miss bad parts,” Trevisani said.

Another dedicated device—for measuring stents—is just now coming on the market from Sensofar (Carefree, AZ). The motivation behind a dedicated device is clear to Ferran Laguarta, president and CEO of Sensofar Medical. “There are over 15 million stents produced worldwide, 5 million in the USA alone and each one needs to be inspected,” he said. Today, that also means a human-intensive process or automation combined with existing metrology equipment that makes for an expensive solution. “We have been very conscious of price while developing our system,” he said.

Their solution is the Q six that combines a high-resolution color camera, three different lighting sources, and an interferometric sensor for 3D measurements. The 2D imaging produces inspections of inner and outer surfaces and the sidewalls of the stents for CD measurements and defect detection and classification. The 3D modes provide surface roughness, surface topography, and coating thickness measurements. The company claims the Q six is fully compliant with USA 21 CFR part 11 reporting requirements. Stent sizes can range from 1.5 to 15 mm in diameter, up to 100 mm in length. Lateral resolution is 0.1 µm and vertical resolution is 0.01 µm.

Their target markets are on-line inspection, process development, and industrial R&D. Laguarta said that the first deliveries of the system are expected to be in June 2014.

Flexibility, Throughput, Uniqueness

Given the wide range of potential biomedical devices and metrology solutions, perhaps there is no simple answer to what is best. This seems to be especially true for metrology systems designed for in-process control. “We offer measuring components such as transducers, measuring arm setups, and air gages. We also act as a gage builder, creating gage fixtures for biomedical devices,” said Gary Sicheneder manager of new business development for Marposs. “What we use truly depends on the application.”

Marposs has delivered metrology systems that measure both biomedical implants and surgical tools used to install them. Other items were check bearings on dentist drills and orthodontic devices. Flexibility is often needed because of the variability required of systems in the biomedical industry. “For example, we conceptualized a system to identify and inspect femoral implants,” he said. They were of various sizes and thicknesses. The contour of each was unique, different from each other. “They needed a flexible system and we quoted a laser-based system that we had adapted from an airfoil inspection system for the aerospace industry we had developed earlier,” he said. Perhaps measuring and inspection in biomedical devices is not so different after all. ME

This article was first published in the May 2014 edition of Manufacturing Engineering magazine. Click here for PDF.